A drive having sintered bearings is shown for example, in EP-A2-0 509 263, which shows a geared motor for a motor vehicle window-lifter drive. As shown, the rotor shaft extends into a gear case that is flange-mounted on the motor frame of a commutor motor, and drives by means of a worm shaft, a worm wheel which is coupled to the cable pulley of a Bowden cable type window-lifter. The rotor shaft is mounted in at least one sintered bearing, which, due to the contact pressures acting upon the flanks of the worm shaft, is also subjected, in particular to radial stress. In geared motor drives of this type, which are made as mass-produced products, the torques developed are subject to a certain scattering. It has been shown that the differences in the starting torques of these geared motor drives are based upon differences in the sintered slide bearings which are used.
An article "Sintered Bearings in Precision Engineering" in the magazine "Fienwerktechnik & Me.beta.technik 98 (1990) 12, pp. 535-538, indicates that sintered bearings display, during run-in, a metallic contact. This is because the porous inlets of the bearing on the running surface are initially open, and hence a large part of the oil in the pressure region of the bearing gap, which is in fact necessary to develop a lubricating wedge, is forced into pore ducts and thus makes the development of a load-bearing oil film more difficult. As a consequence, such bearings operate only in a region of so-called "mixed friction" instead of in a fluid-friction region, which is actually desired.
The above-mentioned operations as to sintered slide bearings can be deduced by applying known laws of hydrodynamic lubrication to solid slide bearings Specific "Stribeck curves", as they are known, can be measured for each type of drive and represent the functional dependence of the bearing friction coefficient ".mu." upon the radial load and upon the rotation speed "n". It can be demonstrated, in particular, that in terms of such curves, the transition from the fluid friction zone into the mixed friction zone and the preceding boundary friction zone is substantially lower in the case of sintered slide bearings than in the case of solid slide bearings.